Coerce matrix.continue-on-error to bool in CI workflow

Since the `matrix.continue-on-error` setting is only ever defined for
nightly Rust build jobs, all other jobs will trigger a CI error if they
fail:

```
Error: Process completed with exit code 101.
Error: .github/workflows/rust.yml (Line: 31, Col: 26): Unexpected value ''
Error: The step failed and an error occurred when attempting to determine whether to continue on error.
Error: The template is not valid. .github/workflows/rust.yml (Line: 31, Col: 26): Unexpected value ''
```

The output above indicates that `matrix.continue-on-error` defaults to
an empty string if it's not explicitly defined for the current job in
the matrix. The `continue-on-error: ${{ matrix.continue-on-error }}`
evaluation then fails due to expecting a `bool` value.

I've tried specifying a default value of `continue-on-error: [false]` in
the matrix, but attempting to override this default setting using
`include` just creates more build jobs with `continue-on-error: true`
instead of overriding the setting for the existing ones, which isn't
what we want.

The solution I found was to append `|| false` to the end, which coerces
the type to `false` if the setting is undefined for the current job.
This suppresses the error message and produces the original expected
behavior.

.github/workflows/rust.yml

@@ -25,8 +25,8 @@ jobs:
         rustup component add rustfmt
         cargo fmt --all -- --check
     - name: Build
-      continue-on-error: ${{ matrix.continue-on-error }}
+      continue-on-error: ${{ matrix.continue-on-error || false }}
       run: cargo build --all --verbose
     - name: Run tests
-      continue-on-error: ${{ matrix.continue-on-error }}
+      continue-on-error: ${{ matrix.continue-on-error || false }}
       run: cargo test --all --verbose